The use of glazes to decorate glasses and ceramics has its roots in antiquity. In essence, glazes are transparent glasses which may be colored or clear which are applied to the surfaces of articles to provide decorative designs and finishes thereto and so enhance the aesthetic appearance thereof. Customarily, glazes are applied to the surface of an article in the form of a slurry or slip of finely-divided glass particles, termed "frit" in the art, and, after drying the slip, the frit is fired at a temperature and for a time sufficient to cause the particles to fuse and flow over the surface to form a continuous glassy coating thereon.
The most widely marketed commercial frits have contained high concentrations of lead oxide (PbO) and, less frequently, substantial amounts of cadmium oxide (CdO). Both of these oxides exhibit two characteristics which render them especially desirable for inclusion in glazing frits. First, they lower the melting point of the glass, thereby enabling it to be fused to flow along the surface of the base body to be coated at a temperature which is sufficiently low to forestall thermal deformation of the base body. Second, they raise the refractive index of the frit, thereby increasing the gloss displayed by the glaze. CdO has also been used as a colorant in certain frits. Nevertheless, because both CdO and PbO are highly toxic, very stringent regulations have been promulgated by the Federal Food and Drug Administration strictly limiting their release when compositions containing those compounds come into contact with comestibles.
Pressure from both the public and private sectors has been continuous and in increasing volume to totally eliminate lead and cadmium from materials which come into contact with food. Therefore, numerous research programs have been initiated in recent years to develop lead- and cadmium-free frits suitable for glazing articles of glass, glass-ceramic, and ceramic.
To perform satisfactorily as a glaze, a frit must satisfy at least the four following basic criteria:
(a) the frit must demonstrate good glass stability; i.e., the frit must not devitrify during the firing of the frit to fuse the glass particles into a flowing mass to coat the surface of the article; PA1 (b) the frit must exhibit excellent resistance to attack by acids and bases to avoid corrosion of the glaze resulting in loss of gloss, the generation of haze and/or iridescence, the development of porosity, or other defects deleterious to the appearance and/or physical character of the glaze; PA1 (c) the firing or maturing temperature of the frit, i.e., the temperature at which the frit demonstrates sufficient flow to yield a smooth homogeneous coating, must be low enough to avert thermal deformation of the article being coated; and PA1 (d) the linear coefficient of thermal expansion of the frit must be compatible with that of the article being glazed in order to avoid crazing and/or spalling, with the preferred frits exhibiting a linear coefficient of thermal expansion somewhat lower than that of the article being glazed, thereby placing the matured coating under compressive stress when the glazed article is cooled to room temperature.
Corning Incorporated, Corning, N.Y., markets a glass-ceramic dinnerware product as Corning Code 0308 under the trademark SUPREMA.RTM.. That product is encompassed within U.S. Pat. No. 4,608,348 (Beall et al.) in that it contains potassium fluorrichterite as the predominant crystal phase with cristobalite as a secondary phase in an amount of about 10-20% by volume and consists essentially, expressed in terms of parts by weight on the oxide basis, of about
______________________________________ SiO.sub.2 66.8 K.sub.2 O 4.75 Sb.sub.2 O.sub.3 &lt;0.1 Al.sub.2 O.sub.3 1.7 Na.sub.2 O 3.3 NiO 0.014 MgO 14.5 Li.sub.2 O 0.8 Co.sub.3 O.sub.4 0.0015 CaO 4.4 P.sub.2 O.sub.5 1.15 Fe.sub.2 O.sub.3 0.02. BaO 0.21 F 3.74 ______________________________________
Because the sum of the recited components closely approximates 100, for all practical purposes the individual values may be deemed to represent weight percent. The precursor glasses for the dinnerware are crystallized in situ to the glass-ceramic state through heat treatment at temperatures of about 950.degree.-1000.degree. C., with the glass-ceramic exhibiting a linear coefficient of thermal expansion over the temperature range of 25.degree.-300.degree. C. between about 120-140.times.10.sup.-7 /.degree.C.
U.S. Pat. No. 5,204,291 (Nigrin) discloses lead- and cadmium-free glazes expressly designed for coating SUPREMA.RTM. dinnerware. The frits therefor consisted essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 51-59 CaO 0-7 Li.sub.2 O 0-2 SrO 0-12 Na.sub.2 O 3.5-7 BaO 0-9 K.sub.2 O 6-8.5 ZnO 0-10 Li.sub.2 O+Na.sub.2 O+K.sub.2 O 10-15 CaO+SrO+BaO+ZnO 8-18 B.sub.2 O.sub.3 9-12 Al.sub.2 O.sub.3 4.5-7. ______________________________________
The frits demonstrated linear coefficients of thermal expansion over the temperature range of 25.degree.-300.degree. C. between about 70-85.times.10.sup.-7 /.degree.C. and good flow at temperatures between about 900.degree.-1000.degree. C. The preferred glaze compositions consisted essentially of
______________________________________ Li.sub.2 O 1-2 SrO 2.5-6.5 Na.sub.2 O 4-7 B.sub.2 O.sub.3 10-11.5 K.sub.2 O 6.5-8 Al.sub.2 O.sub.3 5-6.5 ZnO 7-9.5 SiO.sub.2 55.5-58.5. ______________________________________
Whereas those glazes generally exhibited excellent resistance to attack by acids and bases, good gloss at the firing temperatures, and good compatibility in thermal expansion with the SUPREMA.RTM. ware, they interacted with the SUPREMA.RTM. ware surface during glaze maturing, thereby producing a crystalline interlayer with very large crystals, those crystals comprising mainly magnesium, potassium, zinc, and silicon oxides. The crystals exhibit a roedderite structure and, consequently, demonstrate a low linear coefficient of thermal expansion (estimated to be about 30.times.10.sup.-7 /.degree.C. over the temperature range of 25.degree.-300.degree. C.). Because those large crystals exhibiting such low coefficients of thermal expansion were sandwiched between SUPREMA.RTM. ware with a linear coefficient of thermal expansion of about 120.times.10.sup.-7 /.degree.C. and a glaze with a linear coefficient of thermal expansion of about 80.times.10.sup.-7 /.degree.C., stresses are created in the interface which can weaken the body/glaze bond lead to spalling.
Laboratory experiments have indicated that roedderite crystals are developed only with frits containing more than about 1% by weight ZnO, and that Li.sub.2 O acts as a mineralizer in the system. Removal of those two components, however, raised the melting temperatures of the frits too high to be operable for glazing SUPREME.RTM. dinnerware.
Accordingly, the primary objective of the present invention was to devise CdO- and PbO-free frits which are preferably also free of Li.sub.2 O and ZnO suitable for glazing SUPREMA.RTM. dinnerware.